1. Field of the Invention
Embodiments of the invention relate to a display device, and more particularly to, a liquid crystal display and driving method thereof. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for liquid crystal displays that can be driven according to an impulse driving method.
2. Related Art of the Invention
An active matrix type liquid crystal display (LCD) displays video (or motion picture) by using thin film transistors (TFTs) as switching elements. The LCDs can be fabricated to be compact compared to the cathode ray tubes (CRTs), so the LCDs are being implemented in display devices of portable information devices, office machines, computers, televisions, and the like, rapidly replacing the CRTs.
The LCD has a blurring phenomenon in that a screen image of video is not clear but blurred due to hold properties of the liquid crystal material. In the CRT, phosphors are illuminated only during a very short time, as shown in FIG. 1 to display data on cells and an image is displayed by impulse driving without illumination at the cells. In comparison, in the LCD, as shown in FIG. 2, an image is displayed by a hold driving such that after data is supplied to liquid crystal cells during a scanning period, the data charged in the liquid crystal cells is maintained during the remaining field period (or frame period).
Video (or motion picture) is displayed on the CRT according to the impulse driving, so perceived image that may be viewed by a viewer (observer) is vivid, as shown in FIG. 3. Comparatively, in the LCD, because of the hold properties of liquid crystal material, the contrast of a perceived image that may be seen by the viewer is not clear but dim and blurred. The difference between the perceived images results from the integration effect of an image that temporarily continues in the eyes that follow the movement. Thus, although the response speed of the LCD is fast, the viewer is bound to see the blurred screen image due to the discrepancy between the eyes' movement and a static image of each frame. To avoid such a motion blurring phenomenon in the LCD, a technique for driving the LCD according to the impulse driving method, such as a black data insertion (BDI) method, has been proposed in which after video data is displayed on the screen, black data is provided to the screen.
The black data insertion method is video data being sequentially displayed on the ‘j’ number of lines (‘j’ is a positive integer) in some blocks of the screen, and black data is simultaneously displayed on the ‘k’ number of lines (‘k’ is a positive integer) in other blocks of the screen. Thus, in the black data insertion method, the frequency of data should be fast when the data is displayed on the liquid crystal panel, compared with the frequency of data inputted from the exterior. For this purpose, the frequency (Fi) of a timing signal such as a dot clock inputted from the exterior together with data should be multiplied by
      f    0    =                    j        +        k            j        ·    fi  by using a phase locked loop (PLL) 51 as shown in FIG. 5. A line memory 52 temporarily stores digital video data and then supplies the digital video data to a data driving circuit according to a dot clock having the frequency which has been multiplied by the PLL 51. The PLL 51 and the line memory 52 are positioned in a timing controller to convert a transmission frequency of data because the frequency of the digital video data inputted to the data driving circuit is faster than that of the digital video data inputted to the timing controller. Thus, the related art black data insertion method increases the costs of the timing controller because of the frequency multiplication operation of the PLL 51 and heats up the timing controller. In addition, in the related art black data insertion method, because the operation frequency of the data driving circuit is increased, the heating of the data driving circuit is increased, and also because the transmission frequency of the digital video data is increased between the timing controller and the data driving circuit, EMI (ElectroMagnetic Interference) is also increased.
In an LCD employing such a black data insertion method, the degradation of the charging characteristics of the video data and the black data deteriorates gray scale representation of data and the impulse driving effect. The inventors of the embodiments of the invention conducted experimentation on the LCD such that video data were sequentially displayed on four (j=4) data lines of particular blocks, black data were sequentially displayed, one (k=1) data line (k=1) at a time, at other blocks, and then white gray voltage and black gray voltage were applied to liquid crystal cells of the liquid crystal panel with high driving frequency by multiplying the frequency of dot clock by 5/4*fi. In addition, with the same LCD which, however, did not employ the black data insertion method, the inventors of the embodiments of the invention applied the white gray voltage and the black gray voltage to obtain certain gray scale representation capabilities and data charging characteristics and compared them with those of the LCD employing the black data insertion method. According to the experimentation results as shown in FIG. 6, the normally-driven LCD without employing the black data insertion method had the voltage of liquid crystal cells measured by 4.95V to 50 mV when the data gray scales were changed from the white gray voltage of 255 gray scales to the black gray voltage of 0 gray scale. In comparison, in the LCD employing the black data insertion method, when the white gray voltage of the 255 gray scales was changed to the black gray voltage of 0 gray scale by making the driving frequency fast, the voltage of the liquid crystal cells was measured by 4.95V to 1.04 mV. Thus, in the LCD employing the black data insertion method, when the gray scales of data change from the white gray level to the black gray level, because the black gray voltage is not sufficiently low, the black gray level cannot be properly represented. Also, although there may be a difference to some degree, in the LCD employing the black data insertion method, when data applied to the liquid crystal display panel changes from each gray level to the black gray level, the voltage corresponding to the black gray level is too high to ideally change the data of the black gray level.